Patent Application
20090099409
The present invention relates to a medical sheet made from a super elastic material such as Nitinol for use within a body passageway or duct.
Medical sheets and medical tape are implantable devices and are used to provide structural support within a patient. These support device applications include: slings and organ support sheets, transvascular tape, biological implant materials, fascia and grafts for muscle, tendons, connective tissue, bones, etc. The medical sheets must be flexible and have surface areas that are large enough to provide the necessary support for the internal organ. In order to provide a large flexible surface area, medical sheet and medical tape are frequently a mesh or woven structure.
The present invention is an improved medical sheet for implantation into a patient. The medical sheet has a first smaller surface area in a compressed state which simplifies the insertion into the patient. After the medical sheet is inserted into the patient it is expanded to a larger surface area in an expanded state. The medical sheet is implanted into the patient in the expanded form.
In one embodiment, the inventive medical sheet is a planar member that is rectangular in shape. Amorphic circles are attached to the outer edges of the medical sheet to remove any sharp points. The rounded amorphic circles make the inventive medical sheet a-traumatic to the implanted patient. The amorphic circles have holes which allow for in-growth after the medical sheet has been implanted within the patient. Alternatively, the holes in the amorphic circles can be used as suture points that are used to secure the medical sheet to the desired location within the patient.
The medical sheet may be cut into an intricate pattern of interconnected struts, loops and bridges. Bending of the struts, loops and bridges allows the medical sheet to transform in area between a compressed state and an expanded state. After the medical sheet is fabricated, it can be coated with polymers, therapeutic agents, bioactive materials or radio-opaque materials depending upon the application.
The medical sheet is preferably made from a “super elastic” metal alloy such as Nitinol. Super elastic metal alloys have the physical characteristics of being extremely elastic when in the austenitic molecular phase. In this phase, the inventive medical sheet can be compressed into a small volume without springing back to its expanded shape. In the compressed form, the gaps separating the struts, loops and bridges are very small so that the adjacent struts are in very close proximity to each other. In addition to being compressed in a planar manner, the medical sheet may also be bent or rolled out of plane in an accordion manner to further compresses the medical sheet.
Before the medical sheet is implanted in the desired area within the patient, it is heated to change the super elastic alloy to the austenitic phase causing the medical sheet to expand. The final austenite transition temperature may be about 24° C. to about 37° C. so that exposure to the patient's body heat causes the desired transformation. In the expanded state, the struts, loops and bridges bend so the medical sheet expands to a larger area with larger gaps between the components. After the medical sheet is fully expanded, it has a sufficient area to provide support to the internal organs and can be attached to the patient through in-growth or sutures through the holes in the amorphic circles.
The foregoing and other aspects of the present invention will best be appreciated with reference to the detailed description of the invention in conjunction with the accompanying drawings, wherein:
The present invention is directed towards both a medical sheet and medical tape which only differ in geometry. The medical sheet is rectangular in shape while the medical tape is a thinner and more elongated structure. For simplicity, the present invention is primarily described as a medical sheet. It is intended for the disclosure to be equally applicable to the medical tape embodiment.
To manufacture the inventive medical sheet, a super elastic metal alloy is required. Although the super elastic alloy is described as Nitinol (Ni—Ti alloy), any alloy with similar super elastic properties may be used. Nitinol sheet stock is commercially available from a number of suppliers including Nitinol Devices & Components, Fremont California. Alternatively, the Nitinol sheet stock may be formed through a deposition process such as chemical vapor deposition, physical vapor deposition, and plasma spray deposition. The deposition process builds a layer of material by depositing molecules of the alloy onto a substrate in a vacuum chamber and then the deposited layer is separated from the substrate. The substrate can have various three dimensional shapes, so the vapor deposited Nitinol can also be formed into a planar or three dimensional layer. The thickness of the super elastic alloy medical sheet may range from about 0.0001 to about 0.1 inch.
The Nitinol sheet stock may be cut into the desired fully expanded medical sheet pattern while in the austenitic phase or in the martensitic phase. The phase of the Nitinol material is temperature dependent. In general, the austenitic transition temperature Af is about 24° C. to about 37° C. At temperatures above the austenitic transition temperature, the Nitinol will be in the Austenitic phase. At lower temperatures, the Nitinol may be fully or partially in the martensitic phase. If the medical sheet is cut in the martensitic phase, it can then be maintained in the expanded shape while it is heat treated to convert the Nitinol to the austenitic phase. The medical sheet can then be cooled to transform the medical sheet to the martensitic phase before the medical sheet is compressed into a compact state for implantation in a patient. The austenitic phase shape is the shape that the medical sheet will attempt to assume whenever it is heated above the austenitic transition temperature.
With reference to
In an embodiment, a photochemical etch process may be used to cut the desired pattern into the Nitinol sheet. This process can include various process steps that are generally known as photolithography. A photoresist layer is deposited onto the Nitinol sheet and exposing the photosensitive layer to a pattern of light that matches the desired pattern that the sheet is to be cut into. The light chemically alters the exposed areas of the photoresist layer and a chemical reaction is used to remove the portions of the photosensitive layer that were not exposed to light. An etch process then cuts through the areas of the Nitinol that are not covered by the photoresist to form the patterned medical sheet. The remaining photoresist is removed to produce the finished patterned medical sheet.
In an embodiment, the medical sheet 50 may be made from a plurality of adjacent elongated planar strips 52(a)-52(d) that are secured adjacent to each other across the length of the medical sheet 50 by a plurality of bridges 70. Although four adjacent elongated strips are shown, medical sheets with any number of elongated strips can be made. The edges of the medical sheet 50 that are formed by the ends of the elongated strips 52(a)-52(d) are the retractable sides 43 of the medical sheet 50. The edges of the medical sheet 50 that are formed by the left side of the elongated strip 52(a) and the right side of elongated strip 52(d) are the expandable sides 45 of the medical sheet 50. A longitudinal axis 83 extends between the retractable sides 43 of the medial sheet 50.
The elongated strips 52 each include a plurality of longitudinal struts 60 and a plurality of loops 62 that connect the adjacent struts 60. The adjacent struts 60 are connected with loops 62 in an alternating pattern at opposite ends of the struts 62, forming a serpentine or “S” shaped pattern. In the compressed state, the loops 62 are substantially semi-circular and appear to be about a 180° bend. The space between the struts 60 is very small because the 180° bends of the loops 62 cause the struts 60 to be compressed close to each other.
The medical sheet also includes a plurality of amorphic circles 91 that are attached along the outer edges of the medical sheet 50. Along the short sides of the medical sheet 50, the amorphic circles 91 are attached to the loops 62. Along the long sides of the medical sheet 50, the amorphic circles 91 are attached at a terminal point along the longitudinal length of the outermost strut in the planar strip 52(a)-52(d). The rounded surfaces of the amorphic circles 91 and loops 62 eliminate any sharp external features and make the medical sheet 50 a-traumatic when implanted into a patient. The amorphic circles 91 are round features that may also have a smooth rounded edge, such as a “bull-nose” edge to further remove any sharp surfaces. The amorphic circles 91 will preferably have a rounded ring type shape rather than a cylindrical shape with sharp edges. The radius of the rounded edges will be less than one half the thickness of the Nitinol sheet and preferably less than one quarter of the thickness of the sheet.
Holes 93 in the amorphic circles 91 provide areas for ingrowth to stabilize the medical sheet 50 implanted into a patient. Alternatively, the holes 93 in the amorphic circles 91 may also be used to suture the medical sheet 50 to tissue within the patient. The medical sheet may be used as a physical graft structure or to provide physical support to organs within a patient. The amorphic circles 91 can range in diameters from about 0.001 to about 0.250 inch. The holes 93 are concentric with the amorphic circles 91 and may be proportional in diameter. The diameters of the holes 93 may range from about 10% to about 90% of the diameter of the amorphic circle 91. Although the amorphic circles 91 and holes 93 are illustrated only around the perimeter, the amorphic circles 91 can also be attached to any interior loop 62 of the medical sheet 53 as shown in
In alternative embodiments, some of the amorphic circles 91 may be replaced with rounded structures that provide the same a-traumatic edges to the medical sheet 50 as the amorphic circles 91. These rounded structures may be spheres, ovals, rounded rectangles, rounded triangles, a rounded “T” end, etc. Like the amorphic circles 91, these rounded structures can be attached to the loops 62 anywhere in the medical sheet 50, within any of the bridges 70 or at the struts 60 at the ends of the elongated strips 52(a)-52(d). The rounded structures may also have holes formed through their centers for ingrowth or sutures.
Although the bridges 70 appear to be straight structures connected to loops 62 on adjacent planar strips at an angle as shown in
The geometry of the struts is also designed to better distribute strain throughout the medical sheet and minimize the opening size between the struts, loops and bridges. The number of struts, loops and bridges as well as the design of these components are important factors when determining the working properties and fatigue life properties of the medical sheet. A medical sheet that has a larger quantity of smaller sized struts per elongated strip improves the mechanical properties of the medical sheet by providing greater rigidity than sheets made with fewer and larger struts. For example, a medical sheet where the ratio of the number of struts per elongated strip to the strut length L (in inches) that is greater than 400 has increased rigidity.
After the medical sheet is cut to the desired pattern, surface processing can be performed. The medical sheet may be passivated by exposing the Nitinol to oxygen to form a layer of metal oxide which helps to prevent corrosion. The medical sheet may also be polished to remove any rough surfaces through processes such as: mechanical polishing, electro polishing or chemical mechanical polishing. This polishing removes any sharp surfaces that may have been formed during the medical sheet cutting processes.
Alternatively, the medical sheet may be textured to improve the ingrowth after implantation or improve the adhesion of coatings applied to the medical sheet. The texturing can be through photochemical etching, sand blasting, tumbling, etc. These textured surfaces can then be coated with different materials that will improve the implanted performance. These chemical coatings are generally intended to improve the biocompatibility of the medical sheet within the patient's body by enhancing ingrowth, preventing rejection and resisting infection. These surface coatings include polymers, therapeutic agents and bioactive materials.
In an embodiment, some of the medical sheet may also be coated with a radio-opaque material that is detectable with x-rays. The radio-opaque materials may alternatively be attached to the Nitinol medical sheet by laser welding, adhesives, mechanical fasteners, etc. After the medical sheet has been implanted within the patient, the implant area can be x-rayed to determine the exact position of the medical sheet. If the medical sheet is improperly positioned, the error can be detected and corrected.
After the medical sheet 50 is cut and all surface coatings are applied, it is ready for use. The medical sheet 50 is cooled below the martensitic transformation temperature to change the Nitinol to a super elastic material. The martensitic transformation temperature Mf may be between about 0° to 15° C. In the martensitic phase, the interconnected struts 60, loops 62 and bridges 70 of the medical sheet 50 can be compressed into a small area as shown in
Although the medical sheet 50 is shown in
To implant the medical sheet 50 into a patient, the compressed medical sheet is held by a delivery apparatus and is inserted through a small incision cut through the skin of the patient. After the medical sheet is inserted inside the patient, it is fully expanded before being permanently or temporarily implanted in the patient. The expansion of the medical sheet inside the patient results from a molecular transformation of the metal alloy from the martensitic phase to the austenitic phase which results from the increased temperature inside the patient's body. The patient's body heat converts the phase of the Nitinol material into the austenitic phase. As the molecular structure of the metal alloy changes to the austenitic phase, the medical sheet decompresses into its expanded shape.
With reference to
After being fully expanded inside the patient, the medical sheet 50 is positioned and secured in the patient using other medical instruments. The medical sheet 50 may be attached within the patient's body by ingrowth through the holes 93 in the amorphic circles 91 and the gaps G between the struts 60, loops 62 and bridges 70. Alternatively, sutures may be sewn through the holes 93 to secure the medical sheet 50 in place. After the medical sheet 50 is implanted, all surgical tools are removed so the patient can heal.
As seen from
In contrast, when stress is applied to a specimen of a metal such as Nitinol exhibiting super elastic characteristics at a temperature above which the austenite is stable (i.e. the temperature at which the transformation of martensite phase to the austenite phase is complete), the specimen deforms elastically until it reaches a particular stress level where the alloy then undergoes a stress-induced phase transformation from the austenite phase to the martensite phase. As the phase transformation proceeds, the alloy undergoes significant increases in strain but with little or no corresponding increases in stress. The strain increases while the stress remains essentially constant until the transformation of the austenite phase to the martensite phase is complete. Thereafter, further increases in stress are necessary to cause further deformation. The martensitic metal first deforms elastically upon the application of additional stress and then plastically with permanent residual deformation.
If the load on the specimen is removed before any permanent deformation has occurred, the martensitic phase specimen will elastically recover and transform back to the austenite phase. The reduction in stress first causes a decrease in strain. As stress reduction reaches the level at which the martensite phase transforms back into the austenite phase, the stress level in the specimen will remain essentially constant (but substantially less than the constant stress level at which the austenite transforms to the martensite) until the transformation back to the austenite phase is complete, i.e. there is significant recovery in strain with only negligible corresponding stress reduction. The alloys are structurally stronger and more rigid in the austenitic phase than the martensitic phase. After the transformation back to austenite is complete, further stress reduction results in elastic strain reduction. This ability to incur significant strain at relatively constant stress upon the application of a load and to recover from the deformation upon the removal of the load is commonly referred to as super elasticity or “shape memory.” See for example, U.S. Pat. No. 4,665,905 (Jervis) and U.S. Pat. No. 4,925,445 (Sakamoto et al.).
The transition between martensite and austenite phases can be controlled by the material temperature. The shape material is fully martensitic when it is colder than the final martensitic transition temperature Mf and fully austenitic when the material is heated above the final austenitic transition temperature Af. The alloy may be partially martensitic and partially austenitic at temperatures between the final martensitic transition temperature Mf and the final austenitic transition temperature Af. These shape memory alloys are stronger in the full austenitic phase than in the martensitic state, but no longer have the super elastic property. When a shape memory alloy structure is heated, it reverts, or attempts to revert, to its original heat-stable shape.
The super elastic metal alloys may comprise nickel, titanium and additional elements such as: niobium, hafnium, tantalum, tungsten and gold. The ratio of the nickel and titanium in the super elastic alloy will alter the martensite/austenite transition temperatures. An alloy having more than 50.5 atomic % nickel has a complete transition temperature from the martensite phase to the austenite phase (Af) below human body temperature, so that austenite is the only stable phase at body temperature. The alloy preferably has an Af in the range from about 24° C. to about 37° C. The Mf is about 25 to about 50 degrees C lower than the Af.
Because these super elastic alloys are capable of extreme deformation, it is desirable to design products that will not exceed the maximum allowable strain during use. In trying to minimize the maximum strain experienced by the struts, loops and bridges, the present invention utilizes a structural geometry that distributes strain to areas of the medical sheet which are less susceptible to failure. For example with reference to
It is also desirable to minimize local strain concentrations on the bridge 70 and bridge connection points 72, 74. This can be accomplished at the outset through efficient utilization of materials in the struts 60, loops 62 and bridges 70 increases the strength and the ability of the inventive medical sheet 50 to provide structural support. These strain concentrations can also be minimized by utilizing the largest possible curvature radii in the bridges 70 while maintaining feature widths that are proportional to the applied forces. Another way to minimize the strain concentrations is to minimize the maximum open area between the struts 60, loops 62 and bridges 70 in the medical sheet in the expanded state.
These design characteristics are illustrated in
In a preferred embodiment, there is also a geometric relationship between axial widths of the struts, loops and bridges and the thickness of the medical sheet. With reference to
If the bridges, loops and struts have widths that are greater than the material thickness, they have a greater resistance to in-plane bending than out-of-plane bending. In this configuration, the bridges and struts tend to bend out-of-plane, which may cause a buckling condition that is unpredictable and can cause potentially high strain. These problems have been solved by reducing the width of these features to equal or less than the material thickness.
Nitinol can withstand extremely large amounts of elastic strain deformation, so the above features are well suited to medical sheets made from this alloy. This feature allows for maximum utilization of Nitinol or other material capabilities to enhance radial strength, improve medical sheet strength uniformity, improves fatigue life by minimizing local strain levels and improves medical sheet apposition in irregular organ wall shapes and curves.
Another design feature that improves the uniform expansion of the medical sheet is the angle of the bridges that connect the adjacent elongated sections of the inventive medical sheet. As the medical sheet is transformed from its compressed state to its expanded state, strains are applied to the struts and loops. The forces of the expanding struts and loops are delivered to the bridge ends and alter the angle of the bridges with respect to the loops to which they are connected. As shown in
In an alternative embodiment illustrated in
In addition to changing the direction of expansion, the alignment of the elongated strips will also influence the mechanical properties of the medical sheet in the expanded state. With reference to
According to the description herein, different medical sheets for many implantation applications can be built by altering the lengths and number of elongated sections. The inventive medical sheet can be built for very specific applications including: vaginal slings, fascial slings, suburethral slings, bladder slings and other organ support applications.
Although particular embodiments of the present invention have been shown and described, modification may be made to the device and/or method without departing from the spirit and scope of the present invention. The terms used in describing the invention are used in their descriptive sense and not as terms of limitations.
